Conventional transmissive macroscopic optical elements primarily depend on refraction to control the propagation of light. Refraction relies heavily upon the exact curvature of the surface, and the spatial extent of the element in order to achieve gradual phase accumulation. This imposes a fundamental limitation on the miniaturization of optical sensors and elements, which is necessary for various applications such as the Internet of Things, bio-photonics and two photon absorption microscopy. Metasurfaces, two-dimensional quasi-periodic arrays of sub-wavelength structures, present a method of miniaturizing optical elements. Rather than relying on gradual phase accumulation through light propagation, each sub-wavelength structure imparts a discrete, abrupt change in the phase of incoming light. This has motivated the design of metasurface-based optical elements including lenses, focusing mirrors, vortex beam generators, holographic masks, and polarization optics.
Thus far, high quality metasurface optical elements based on metals, titanium oxide, and amorphous silicon have been demonstrated. Unfortunately, metals are significantly lossy at optical frequencies, titanium oxide lacks CMOS compatibility, and amorphous silicon absorbs light in the visible and near-infrared spectrum (˜400-900 nm). This wavelength range is of particular interest for many applications due to ubiquitous, low-cost silicon detectors, motivating the development of high band gap material based metasurfaces. However, high band gap CMOS-compatible materials such as silicon nitride and silicon dioxide, which are transparent over the aforementioned wavelength range, have a low refractive index. Due to the low refractive indices of these materials, conventional metasurface design principles fail to apply when forming optical elements.
Accordingly, new metasurface designs and compositions are needed in order to access high band gap CMOS-compatible materials as elements of metasurface-based optics.